Apparatus for measuring the positions of plural movable members each associated with a respective magnetorestrictive element

ABSTRACT

Apparatus for indicating the value of a variable comprising first and second members moveable relative to each other as a function of said variable, a plurality of elements of a first type on one of said members, at least one element of a second type on the other of said members, one of said types being magnetostrictive and the other being operative to produce a magnetic bias field, said elements being arranged so that as said first and second members move relative to each other different ones of said plurality of elements of said first type magnetically interact with said at least one element of said second type such that, for different positions of said members, said interaction causes magnetostrictive resonance at different frequencies in response to an alternating interrogating magnetic field at said different frequencies.

RELATED APPLICATION/PATENT

This invention is related to our application Ser. No. 08/075,582 filedJun. 15, 1993 now issued U.S. Pat. No. 5,420,569 (based on PCT/GB92/00014 filed Jan. 3, 1992 and designating the United States).

RELATED APPLICATION/PATENT

This invention is related to our application Ser. No. 08/075,582 filedJun. 15, 1993 now issued U.S. Pat. No. 5,420,569 (based on PCT/GB92/00014 filed Jan. 3, 1992 and designating the United States).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to devices and apparatus for indicating a variablesuch as the position of a movable member. For example, the invention maybe applied to the determination of the position of the dials in a metersuch as a water or gas meter or to linear or to rotary encoders.

2. Related Art

The prior art contains a large number of devices and proposals formeasuring the relative positions of two members. One proposal, asdisclosed in European Patent Application Nos. 240707 and 369493,involves providing a magnet on one of the members and an array ofmagnetically sensitive resistors on the other, that is to say resistorswhose resistance changes in response to the application of a magneticfield thereto. The arrangement is such that as the two members moverelative to each other, the magnet moves over the array of resistors.Position is determined by a measuring circuit connected to the array ofresistors and operative to detect which resistor has its resistanceaffected by the magnetic field created by the magnet. The problem withthis proposal is that electrical circuitry has to be physicallyconnected to all of the resistors in order to determine the relativepositions of the two members. Such circuitry is therefore relativelycomplex and expensive.

Another proposal, as disclosed in UK Patent Application No. 2016694,employs a magnetostrictive rod effectively operating as an ultrasonicdelay line. The rod is attached to one of the movable members and anelectrical transmitter coil surrounding the rod and moveable therealongis attached to the other. Application of an alternating pulse to theelectrical coil will create an alternating field which will set up anultrasonic wave in the magnetostrictive rod. Pick up coils arepositioned at each end of the rod and connected to an electrical circuitfor determining the difference in time of arrival of the ultrasonic waveat the two ends of the rod. This time difference is indicative of theposition of the moveable coil with respect to the length of the rod.This proposal again requires electrical circuitry connected to both thetransmitter and the pick up coils for determining relative position.

In a further proposal, disclosed in U.S. Pat. No. 4,710,709, one of therelatively moveable members carries two overlapping electrical coils andthe other a bar which is moveable axially through the coils and includesa cavity containing a magneto strictive element which is free to vibratemechanically within the cavity. The first mentioned member also carriesmeans for generating a DC bias magnetic field to which themagnetostrictive element is subjected. As the bar moves through thecoils, the strength of the DC bias field on the magnetostrictive elementvaries and causes the Young's modulus of the element and therefore itsresonant frequency to vary also. Change in resonant frequency istherefore a measure of change in position and is detected by applying aninterrogating field to the magnetostrictive element by energizing one ofthe overlapping coils with a signal of appropriate frequency, which maybe swept through the range of frequencies at which the magnetostrictiveelement may resonate. Upon resonance, the magnetostrictive elementgenerates an alternating magnetic field at its resonant frequency, whichis detected by the second of the overlapping coils. The problem withthis proposal is that the range of displacements which may be detectedis limited by the distance through which the magnetostrictive elementmay move relative to the DC field generating means to produce adetectable variation in resonant frequency.

SUMMARY OF THE INVENTION

With a view to overcoming or alleviating the above problems, one aspectof the present invention comprises first and second relatively moveablemembers, a number of magnetostrictive elements on one of the members andadapted to resonate at respective different frequencies, and means onthe other of said members for producing a magnetic biasing field suchthat different said magneto strictive elements are subjected to saidbiasing field dependent upon the relative position of the first andsecond members. The biasing field is such that the magnetostrictiveelements subjected thereto will mechanically resonate in response to aninterrogating AC magnetic field of appropriate frequency and thusproduce a detectable AC magnetic field. Since the magnetostrictiveelements resonate at different frequencies, the detected frequency willbe indicative of the relative position of the first and second members.In a preferred form, the biasing means is such that differentcombinations of said magneto strictive elements are biased dependentupon the relative positions of the first and second members according toa predetermined code and thus the different combinations of frequenciesproduced by the interrogating field will be indicative of the relativepositions of the first and second members.

In accordance with an alternative embodiment, position detectingapparatus comprises first and second relatively moveable members, amagnetostrictive element on one of the members, and magnetic biasingmeans on the other of the members, the magnetic biasing means beingoperable to produce different DC magnetic field patterns on saidmagnetostrictive element in dependence upon the relative position of thetwo members, said field patterns biasing said magnetostrictive elementto resonate at one or more of its fundamental frequency and harmonicsthereof in response to an applied interrogating AC field of appropriatefrequency.

In a further aspect, the invention provides a remotely readableindicator comprising magneto strictive means, means for producing amagnetic field for biasing the magnetostrictive means and means forvarying the relationship between the magnetostrictive means and thebiasing field and/or for varying the bias field as a function of avariable or quantity to be measured or indicated so that themagnetostrictive means is responsive to interrogating alternating fieldsof different discrete frequencies dependent upon the value of thevariable or quantity.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described further, by way of example, with reference tothe accompanying drawings in which:

FIGS. 1 to 9 are diagrams for illustrating theelectromagnetic/magnetostrictive principles utilized in the preferredembodiments of the invention;

FIG. 10 diagrammatically illustrates a remotely readable meter, such asa gas meter, comprising six dials, according to an embodiment of theinvention;

FIG. 11 is a perspective view of a dial of the meter of FIG. 10 showingmore detail;

FIG. 12 illustrates by way of example, patterns of magnetisation whichmay be provided on the dials of FIG. 10 in accordance with an aspect ofthe invention;

FIG. 13 illustrates graphically examples of responses which may beobtained from a given dial of FIG. 10 as the dial rotates to differentsuccessive positions:

FIG. 14 illustrates the size relationship between respective differentresonant strips associated with respective different ones of the dialsin the meter of FIG. 10;

FIG. 15 is a diagram illustrating an arrangement for remotely readingthe meter of FIGS. 10 to 14;

FIG. 16 is a diagrammatic perspective showing an alternative form ofdial arrangement;

FIG. 17 diagrammatically illustrates part of the patterns ofmagnetisation provided on the dials of FIG. 16;

FIGS. 18 and 19 are similar to FIGS. 16 and 17, but showing a furtheralternative dial arrangement;

FIGS. 20 and 21 are also similar to FIGS. 16 and 17 and showing a yetfurther alternative arrangement of dials;

FIG. 22 illustrates a further embodiment, in which increased accuracymay be achieved;

FIG. 23 is a partial side view, partially in section, showing a furtherembodiment of the invention;

FIG. 24 is a perspective view of part of the embodiment shown in FIG.23;

FIG. 25 is a partial section on the line X--X of FIG. 23;

FIG. 26 shows in more detail an array of magnetostrictive elementsutilized in the embodiment of FIGS. 23 to 25;

FIG. 27 is an enlarged partial perspective view partly cut away of adial included in the embodiment of FIGS. 23 to 26;

FIG. 28 is a view similar to FIG. 23 showing a modification to theembodiment of that Figure;

FIG. 29 is an end view of the apparatus shown in FIG. 28.

FIG. 30 is a diagrammatic perspective view partly cut away showing afurther embodiment of the invention;

FIG. 31 is a diagram illustrating the operation of the embodiment ofFIG. 30;

FIG. 32 is a block diagram of a control and display unit of theembodiment of FIG. 30;

FIG. 33 is a diagram illustrating a modification to the embodiment ofFIG. 30;

FIG. 34 is a perspective view partly cut away of a further embodiment ofthe invention;

FIG. 35 is a diagram illustrating the operation of the embodiment ofFIG. 34;

FIGS. 36 to 38 are diagrams showing further possible shapes for themagnetostrictive resonant element employed in the present invention;

FIG. 39 is a diagram showing a set of magnetostricitive elements similarto those illustrated in FIGS. 36 to 38 but formed from a unitary pieceof material;

FIG. 40 diagrammatically illustrates a modification to the embodimentshown in FIG. 30;

FIG. 41 diagrammatically shows a modification to the embodiment of FIGS.23 to 27;

FIG. 42 is a diagrammatic perspective view illustrating a modificationwhich may be made to the embodiments of FIGS. 16 to 29;

FIG. 43 is a diagrammatic plan view illustrating a modification wherebythe response of the magnetostrictive resonant elements to aninterrogating field may be enhanced;

FIG. 44 is a diagrammatic side view of the modification of FIG. 43;

FIG. 45 is a diagrammatic plan view showing a further modificationwhereby the response of the magnetostrictive resonant element to aninterrogating field may be enhanced;

FIG. 46 is a diagrammatic side view of the modification of FIG. 45; and

FIGS. 47 to 52 are diagrams illustrating the creation of different modesof vibration in the magnetostrictive element.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTSElectromagnetic/Magnetostrictive Principles

FIG. 1 shows a magnetostrictive element 12 in the form of a strip ofmagnetostrictive material disposed adjacent a rectangular plate 14 ofhard material magnetized to act as a magnetic biasing element for themagnetostrictive strip 14. Thus, the plate 14 has south and north poles24, 26 respectively at each end and produces a magnetic field to whichthe element 12 is subjected as indicated by the arrows.

As is known, with the biasing element 14 magnetised in the same way as asimple bar magnet, i.e a north pole at one end and a south pole at theother as illustrated in FIG. 1, the magnetostrictive element 12 would bestressed by the resulting magnetic field in such a way that if theelement 12 were subjected to an external interrogating alternatingmagnetic field at a frequency equal to the natural frequency of theelement 12, that element would mechanically vibrate and produce adetectable regenerated alternating magnetic field having the samefrequency. This effect can be understood from consideration of FIG. 2which comprises two curves illustrating the way in which the sensitivityof the element 12 to the applied interrogating field varies as afunction of the strength of the biasing field produced in the strip 12by the magnetisation of the biasing element 14. In FIG. 2, curve A is aplot of the strain produced in a magnetostrictive element against fieldH applied to the element. Thus, at the origin, where the field H iszero, there is no strain. If the field H is increased to a value S or-S, the magnetostrictive element becomes saturated and further increasein the field (in either direction) does not produce any further strain.Curve B shows that the sensitivity of the device to the applied fieldincreases linearly with increasing strength of the bias field H and thusthe strength of the magnetism applied to the element 12 is chosen toprovide a biasing field towards the upper end of the sensitivity curve.For example the fields represented by arrows 28 in FIG. 1 might have avalue H1 or -H1 as shown in FIG. 2.

With the simple pattern of magnetization in element 14 shown in FIG. 1i.e. a north pole at one end and a south pole at the other end, element12 will resonate at its fundamental or natural frequency, as alreadyexplained, in response to an applied interrogating field at thatfrequency and this phenomenon, as will be described later, is utilizedin certain embodiments of the invention. In other embodiments of theinvention, as will also be described later, a magnetostrictive elementis caused to resonate at one or more of the harmonics of its natural orfundamental frequency in response to interrogating fields havingfrequencies equal to such harmonics. To achieve this resonance, morecomplex magnetic patterns than that shown in FIG. 1 are provided in thebiasing element 14. Such patterns may, as shown in FIG. 3, be recordedin the biasing element 14 by transporting it past but in close proximityto a magnetic recording head 18 of conventional type, as shown by thearrow. As the element 14 is transported past the recording head 18, asignal generator 20, controlled by a control unit 22, energises therecording head 18 with a signal whose waveform is selected to producethe required magnetic pattern in the biasing element 14.

FIG. 4 illustrates an example of a waveform for application to the head18 for storing a magnetic pattern in the element 14 which will bias themagnetostrictive element 12 in such a way that it will resonate at afrequency of twice its natural or fundamental frequency in response toan applied interrogating alternating field also having a frequency oftwice the natural or fundamental frequency of the strip 12. As can beseen in FIG. 4, the waveform is a sine wave 15 whose frequency and phaserelative to the movement of the element 14 past the recording head 18are chosen so that, when the elements 12 and 14 are positioned as shownin FIG. 1, a single cycle of the sine wave as recorded in element 14coincides with the length of the magnetostrictive element 12 with thezero crossing points 17 and 19 of the sine wave substantially coincidentwith the ends 12a and 12b of the element 12. In the recording process,the signal source 20 is turned on prior to arrival of the element 14beneath recording head 18 and is turned off after the element 14 haspassed the recording head 18 so as to avoid the generation of transientsin the recording thereof in the element 14, which may arise if thesignal source 20 were turned on and off at the zero crossing points 17and 19. This process is illustrated by dotted line portions 21 and 23 ofthe sine wave 15 in FIG. 4.

As can be seen in FIG. 5, the resulting magnetic pattern in element 14comprises south pole regions 24 near to the centre of the strip andnorth pole regions 26 towards the ends of the strip 14. Arrows 28 and 30in FIG. 5 indicate the magnetic lines of force arising from the abovedescribed magnetic pattern recorded in element 14 and, as shown, by thedirection of these arrows, the resulting field with which themagnetostrictive element 12 is biased is directed to the right in theleft hand portion of the strip 12 and to the left in the right handportion. Accordingly, when the element 14 is subjected to aninterrogating alternating magnetic field of twice the natural frequencyof the strip 12, the two halves thereof will resonate in phaseopposition to each other at a frequency equal to twice the naturalfrequency. Such resonance, which is a mechanical vibration, will producea detectable regenerated magnetic field at twice the natural frequencyof the strip 12.

If it were desired to cause the magnetostrictive element 12 to resonateat a frequency of three times its natural frequency in response to anapplied interrogating field of corresponding frequency, the waveformshown in FIG. 6 may be used when performing the programming illustratedin FIG. 3. As shown in FIG. 6, the sine signal 25 applied to therecording head 18 as the element 14 is moved therepast is at a frequencyand phase relative to the movement of the element 14 such that one and ahalf cycles of the sine wave applied to recording head 18 correspondsubstantially to the length of the strip 12 with zero crossing points 27substantially coincident with the ends of the strip 12. As in theembodiment of FIG. 4, the sine signal 25 is turned on prior to arrivalof the element 14 beneath the recording head 18 and off subsequent tothe departure of the element 14 from beneath the recording head 18 forthe same reasons.

The magnetic field produced in the strip 12 by the element 14 with thesignal 25 recorded thereon comprises three portions 32, 34 and 36. Theportion 32 in the left hand one third of the element 12 is directed tothe right, the portion 34 in the centre one third of the element 12 isdirected to the left and the portion 36 in the right hand one third ofthe element 12 is directed to the right. Such strip will, accordingly,resonate at three times its natural frequency in response to an appliedalternating magnetic field having a frequency three times the naturalfrequency of the strip. Such resonance, which is a mechanical vibration,will produce a detectable regenerated magnetic field at three times thenatural frequency.

FIG. 7, which is similar to FIG. 6, shows at 40 a sine wave which may beapplied to the recording head 18 to produce resonance at four times thenatural frequency of the magnetostrictive element 12. The frequency ofthe signal applied to the recording head 18 is such that two full cyclesof sine wave are recorded in element 14 with zero crossings arranged aspreviously described and the recording of transients being avoided aspreviously described. As shown at 42, 44, 46 and 48, the resultingbiasing magnetic field pattern to which the strip 12 is subjectedcomprises four zones in which the direction of the field reverses formone zone to the next. Thus, the strip will resonate mechanically inresponse to an applied interrogating magnetic field of four times thenatural frequency of the strip, the frequency of resonation being fourtimes the natural frequency of the strip. Again, this produces adetectable regenerated field at four times the natural frequency of thestrip.

The element 14 can be programmed so that the magnetostrictive element 12will resonate at higher harmonics by correspondingly adjusting thesignal recorded by the recording head 18. Further, although up to thispoint, description has been given of how to cause resonance at only asingle frequency which is a harmonic of its fundamental frequency, it ispossible to program the element 14 so that the magnetostrictive element12 will be capable of resonating in response to each of a number ofinterrogating frequencies. Such frequencies may comprise the fundamentaland one or more harmonics or the fundamental may be omitted, as desired.This is simply achieved by recording a magnetic pattern on the strip 14which represents the superposition of the magnetic patterns necessaryfor each of the individual frequencies required. An example of this isshown in FIG. 8 which shows at 50 a waveform for recording on the strip14 to cause resonance of the magnetostrictive element 12 at both twiceand four times the fundamental frequency. The waveform 50 is derived bysimply adding waveforms 15 and 40 which are respectively the same as thewaveforms shown in FIGS. 4 and 7.

Although FIG. 8 illustrates encoding for resonance at two harmonics, itis possible to encode for resonance at more than two harmonics simply byadding together the waveforms for the required harmonics, or to encodefor resonance at the natural frequency of the element 12 plus one ormore harmonics by adding together the waveform required for thefundamental (which would be half a cycle of sine wave with the zerocrossing points substantially coincident with the ends 12a, 12b of thestrip 12) and that required for each desired harmonic. The amplitude ofthe resonance produced at different frequencies is a function, interalia, of the amplitude of the signal recorded on the element 14 toproduce the resonance. Accordingly, the recorded signals for producingdifferent resonances may have different amplitudes to compensate forother factors in the system such as the fact that the amplitude of theresonance at higher harmonics tends naturally to be less than that atlower harmonics. Examples of other factors which may be compensated forin this way are noise, receiver sensitivity at different frequencies,differing interrogation field strengths at different frequencies etc.

When adding together waveforms to obtain resonance at a number ofdifferent frequencies, the amplitude and phase relationships between thesignals to be added should be selected to avoid saturation of themagnetostrictive element 12. An example of how this may be achieved isshown in FIG. 9 in which waveform 15 (which is the same as previouslydescribed) is to be added to a waveform 57 which, as will be appreciatedfrom consideration of FIG. 9, will provide resonance at six times thenatural frequency of the strip 12. The preferred phase of waveform 57 isshown in full lines in FIG. 9 and it is seen that the peaks 59 and 61which are coincident with the peaks of waveform 15 are of opposite signto the corresponding peaks in waveform 15, thus avoiding addition of thepeaks of the two waves. A broken line wave shown at 57a in FIG. 9 is theinversion of waveform 57 and its phase is thus such that its peaks wouldadd to the peaks of waveform 15. In those circumstances, the amplitudesof the two waves would have to be kept to a lower level than wherewaveform 57 is used if saturation of the magnetostrictive strip 12 is tobe avoided.

Embodiments of the Invention

A number of embodiments of the invention in meters, such as gas or watermeters, for the reading of the positions of the dials thereof will nowbe described with reference to FIGS. 10 to 29. In the embodiments ofFIGS. 10 to 23, dial positions are indicated by causing magnetostrictiveelements to resonate at harmonics of their fundamental frequency inaccordance with the principles explained above with reference to FIGS. 4to 9. In the embodiments of FIGS. 24 to 29, a number of magnetostrictiveelements are provided each having a different fundamental frequency, andthese are caused to resonate in different combinations at theirrespective fundamental frequencies for indicating dial position.

FIGS. 10 to 15 illustrate a remotely readable meter 101 in which theinvention is embodied. As shown in FIG. 10, the meter comprises aconventional sensing mechanism 100 (not shown in detail) for example forsensing gas or water flow, and six dials 102, 104, 106, 108, 110 and 112driven by the sensing mechanism through a conventional linkage which isnot shown but is diagrammatically respresented by broken lines 114. Ascan be seen in FIG. 10, each of the dials 102 to 112 is marked with thedigits 0 to 9 around its face and a casing 116 positioned adjacent toeach dial bears a mark 118 indicating the current value represented bythe rotary position of the dial. As is conventional, the six dialsrepresent respectively the digits of a six digit number and are mountedrotatably in a housing (not shown) to which the casings 116 are alsoattached.

The dial 102 and casing 116 are shown in perspective in FIG. 11 and ascan be seen the casing 116 is rectangular and is parallel to the rotaryaxis 120 of the dial and is substantially co-extensive with the axiallength of the dial. The casing 116 contains a rectangularmagnetostrictive strip element 122 which is similar to the element 12 ofFIG. 1 and is contained within a rectangular cavity inside casing 116with clearance so that the element 122 may mechanically vibrate. Thecasing 116 is wholly made of a magnetically transparent material, suchas a synthetic plastics material.

As is clear from FIGS. 10 and 11, each dial 102 to 112 is in the form ofa drum. Strips 124 (similar to the element 14 of FIG. 1) of hardmagnetic material are attached to the periphery of the drum and extendlongitudinally thereof. Each strip 124 is aligned with a correspondingnumber on the face of the dial and has recorded thereon a pattern ofmagnetism which is such that when a given strip is adjacent to thehousing 116, the resulting magnetic field pattern will bias the strip122 such that it will resonate at a particular frequency. FIG. 12illustrates examples of the magentic field patterns which may beproduced by the respective different strips 124. Thus, the strip 124adjacent the number "1" on the dial produces a magnetic pattern whichwill cause the magnetostrictive element 122 to resonate at itsfundamental or natural frequency f in response to an interrogating fieldhaving a frequency f. Similarly, the strips 124 adjacent the numbers 2to 5 on the dial have magnetic patterns recorded in them which are suchthat the magnetostrictive element 122 will resonate at frequencies 2f to5f in response to interrogating fields of frequencies 2f to 5frespectively. As shown in FIG. 12, the magnetic field patternsassociated with the numbers 6 to 9 and 0 are also such as to produceresonances within the range f to 5f. However, as represented in FIG. 12,the width of the magnetostrictive element 122 is greater than the widthof each strip 124 and thus the magnetostrictive element 122 is alwaysinfluenced by either two or three of the strips 124. Thus, although themagnetostrictive element 122 is resonant at 4f both at dial positionnumber 4 and dial position number 7, for example, it will when atposition number 4 also resonate at 3f and 5f whereas when at positionnumber 7 it will resonate also at 2f and f. In this way, position 4 andposition 7 can be distinguished from each other. This is furtherillustrated in FIG. 13 which shows the resonances produced as the dialmoves from position 3 to position 4. Thus, curve A in FIG. 13illustrates the resonances produced at position 3, these being 3f withhigh amplitude and 2f and 4f with relatively low amplitude. Curve B inFIG. 13 illustrates the resonances produced as the dial is approximatelyhalf way between positions 3 and 4, i.e. approximately equal amplituderesonances at 3f and 4f are produced whereas there are low amplituderesonances at 2f and 5f. Curve C in FIG. 13 illustrates the resonancesproduced when the dial is at position 4, namely high amplitude resonanceat 4f and approximately equal but low amplitude resonances at 3f and 5f.As will now be clear, the coding arrangement for the digits shown on thedial is as follows:

                  TABLE 1                                                         ______________________________________                                        DIGIT     RESONANT FREQUENCIES                                                ______________________________________                                        0         3f, 5f, f                                                           1         5f, f, 2f                                                           2         f, 2f, 3f                                                           3         2f, 3f, 4f                                                          4         3f, 4f, 5f                                                          5         4f, 5f, 2f                                                          6         5f, 2f, 4f                                                          7         2f, 4f, f                                                           8         4f, f, 3f                                                           9         f, 3f, 5f                                                           ______________________________________                                    

In the above table f is the natural frequency of the magnetostrictivestrip. This type of coding arrangement has the advantage that the numberof frequencies used at each dial is minimised.

Thus, to interrogate the dials, an interrogating alternating magneticfield is applied and the frequency thereof is swept through the requiredrange of frequencies. So that one dial can be distinguished fromanother, the lengths of the magnetostrictive strips adjacent therespective different dials are different as illustrated in FIG. 14.Thus, strips 122A to 122F correspond respectively to dials 102 to 112and each has a different fundamental frequency and therefore a differentset of harmonics.

FIG. 15 shows an interrogation arrangement for the meter 101. Thiscomprises a first coil 130 positioned adjacent the meter, a second coil132 remote from the meter but connected to the coil 130 by simpleelectrical conductors 134 and a portable reading device 13, which may becarried by the person whose job it is to read the meter. Device 13comprises a transmitting and receiving coil 136, a power supply 138therefor, a control unit 140 for driving the power supply to cause thecoil 136 to produce an alternating magnetic field whose frequency isswept or stepped through the range of frequencies (including theharmonics) at which the magnetostrictive elements 122 may resonate, adecoder 142 for decoding the detected regenerated fields and a datastore 144 into which data read from each meter is stored, together withthe identity of the meter, under control of the control unit 140.

FIG. 16 shows a modified meter arrangement. In this embodiment the dials102' to 112' are mounted on a common axle 160 and the dial numbers areon the periphery rather than on the end face and visible through anapertured window plate 162. The numbers on the dial face are marked onhard magnetic strips 124 which, as shown in FIG. 17, have recordedthereon magnetic patterns similar to those shown in FIG. 12.Magnetostrictive resonator elements 122'A to 122'F are, as in FIG. 10,located adjacent the peripheries of the dials. However, in this case,the axial lengths of the dials and thus the lengths of themagnetostrictive strips and of the hard magnetic strips are shorter thanin FIG. 10.

The embodiment of FIG. 18 and 19 is similar to that of FIGS. 16 and 17except that magnetostrictive strips 124" are provided on the side facesof the dials instead of their peripheries as shown best in FIG. 19, 20and the magnetostrictive resonators 122" are positioned adjacent theside faces as shown in FIG. 22. The resonators 122" and strips 124"extend radially in FIGS. 18 and 19.

In FIGS. 20 and 21, the arrangement is similar to that shown in FIGS. 18and 19 but the magnetostrictive strip elements 122'" extend along achord of the side faces of the dials 102' to 112' and a hard magneticdisc is provided on the side face of each dial and magnetic patterns arerecorded thereon as shown in FIG. 21. In that Figure, shaded areasindicate that the direction of the field is clockwise and unshaded areasindicate that the direction of the field is anticlockwise. Thus, it willbe readily appreciated that the adjacent strips 122'" may be biased toresonate at selected frequencies as in the previous embodiments.

FIG. 22 illustrates a modification to the dial arrangement of FIG. 10for increased accuracy. The arrangement of FIG. 22 is the same as thatof FIG. 10 except that the dial has associated therewith an additionalmagnetostrictive resonator 123 contained in a casing 117. Thearrangement of the resonators 122 and 123 relative to the dial 102 issuch that when one of the numbers on the dial and therefore one of thestrips 124 is aligned with one of the resonators, the other resonator isapproximately half way between two of the adjacent strips 124. So thatthe resonances from the two strips 122 and 123 can be distinguished fromeach other, they are preferably of different lengths. By appropriatedecoding of the signals regenerated by the two strips, accuratedetermination of the position of the dial can be made. Although FIG. 21only shows a single dial, the same arrangement can be provided on eachof the other dials in the meter with of course different frequencies forthe different strips.

With reference to FIGS. 23 to 26, an indicator 200 of a meter, such as awater or gas meter (not shown) comprises a set of six dials 202, 204,206, 208, 210, 212 mounted coaxially on a support structure 214 which inturn is attached to a casing 216 (only partially shown) containing thegas or water flow sensing arrangement of the meter. The dials 202 to 212are rotatable about their common axis 218 and are interconnected anddriven by means not shown so as to form a six decade counter. As shown,each of the dials has the numerals 0 to 9 marked on its periphery and awindow 220 is provided in the support structure 214 so that the positionof the dials can be visually inspected for reading the meter, for whichpurpose a cover 222 enclosing the structure 214 is transparent.

To enable the meter to be read electromagnetically, an array 224 ofmagnetostrictive elements is positioned adjacent the dials 202 to 212and each dial 202 to 212 carries two members 226a, 226b which are ofhard magnetic material and are magnetized for applying DC bias fields tothe magnetostrictive elements in the array 224. The arrangement is suchthat the magnetostrictive elements in the array 224 all have differentfundamental frequencies and different combinations of magnetostrictiveelements are biased for different meter readings.

Thus, the array 224 comprises 6 sets of magnetostrictive elements, thesets being indicated respectively by reference numbers 228, 230, 232,234, 236 and 238. Each set is positioned adjacent a respective differentone of the dials 202 to 212. Each set 228 to 238 comprises 4magnetostrictive elements 228a to d, 230a to d etc. The magnetostrictiveelements are of each set are interconnected with each other and withsupport plates 240 and 242 by ligaments 244. Conveniently, the array224, plates 240 and 242 and ligaments 244 may be formed by etching asheet of magnetostrictive material. The support plates 240 and 242 areclamped to support bars 246 and 248 respectively which in turn arecarried by the structure 214 so that, as best seen in FIGS. 24 and 25,the array 224 is mounted in a curved configuration concentric with thedials and closely adjacent the peripheries thereof with each set 228 to238 of magnetostrictive elements adjacent the respective associated dial202 to 212. The arrangement is such that each set 228 to 238 ofmagnetostrictive elements extends round part of the circumference of itsassociated dial and each magnetostrictive element 228a to d to 238a to dextends axially with respect to its associated dial 202 to 212. Themagnetostrictive elements 228a to d etc of each set are spaced so as tobe in register with four successive numbers on the dial periphery foreach of the ten integer positions of the dial.

The hard magnetic element 226a, which is a rectangular piece of materialbent into an arc and positioned in an arcuate slot 250 in the dial,extends circumferentially over two adjacent digit positions of the dial,as best seen in FIG. 25 and 27.

The hard magnetic element 226b, which is a rectangular piece of materialbent into an arc and located in an arcuate slot 252 in the dial, extendsover four adjacent digit positions as also best seen in FIG. 25. Thus,by way of example, hard magnetic element 226a may extend over digitpositions 8 and 9 and hard magnetic element 226b may extend over digitpositions 3 to 6. Digit positions 0, 1, 2 and 7 do not have a hardmagnetic element associated with them. With this arrangement, differentcombinations of the four magnetostrictive elements 228a to 228d etc foreach dial will be magnetically biased by the elements 226a and 226b foreach different digit position of the dial. The elements 226a and 226bare each magnetized so that their north and south poles N,S are spacedapart axially of the dials, i.e. the north and south poles N,S run alongthe respective longitudinal edges of the elements 226a and 226b andcreate fields which are directed axially of the dials 202 to 212. Thearrangement is such that the DC biasing field direction experienced bythe magnetostrictive elements of each set 228 to 238 is as shown by thearrows 254, 256, 258, 260, 262 and 264 in FIG. 26. Thus, the directionof biasing is longitudinally of each magnetostrictive element (as isshown in the diagram of FIG. 1) so that each element will resonatelongitudinally (i.e axially of the dials) in response to an appliedalternating magnetic field having a frequency corresponding to itsfundamental frequency of mechanical vibration. As shown by the arrows254 to 264 in FIG. 26, alternate biasing fields 254, 258 and 262 aresuccessively in opposite directions to each other. This also applies toalternate biasing fields 256, 260 and 264. This arrangement ensures thatthe alternate biasing fields (say 254 and 258) will not add to causeunwanted biasing of the intermediate set of magnetostrictive elements(say 230).

Each of the twenty-four magnetostrictive elements in the array 228 has adifferent length and thus a different fundamental frequency. As a resultof this and of the interaction between the magnetostrictive elements andthe hard magnetic elements 226a and 226b provided in each dial, a codeis provided whereby a unique combination of frequencies for each of thenumbers from 0 to 999,999 will be generated upon application of analternating magnetic field containing or swept through all of thefundamental frequencies of the magnetostrictive elements. This will bemore fully understood from consideration of the following table whichillustrates the coding of a single dial. The left hand column indicatesthe digit positions 0 to 9 and the columns headed a to d correspondrespectively to the four magneto strictive elements asssociated withthat dial (for example 228a to 228d). The binary digits "0" or "1" ineach of the columns a to d indicate whether or not a respectivemagnetostrictive element is biased by the hard magnetic element 226a or226b. Binary "0" indicates that the element is not biased and willtherefore will not resonate in response to the interrogating field andbinary "1" indicates that the element is biased and therefore willresonate in response to the interrogating field.

                  TABLE 2                                                         ______________________________________                                                   MAGNETO STRICTIVE ELEMENT                                          DIAL POSITION                                                                              a        b        c      d                                       ______________________________________                                        0            0        1        1      1                                       1            1        0        1      1                                       2            1        1        0      1                                       3            0        1        1      0                                       4            0        0        1      1                                       5            0        0        0      1                                       6            1        0        0      0                                       7            1        1        0      0                                       8            1        1        1      0                                       9            1        1        1      1                                       ______________________________________                                    

In the embodiment shown in FIGS. 23 to 27, electro magnetic reading ofthe meter is achieved in the manner described with reference to FIG. 15for which purpose, a coil 130 is mounted on the housing 222. In thisembodiment, the coil 130 comprises a ferrite rod 270 with a winding 272therealong, the winding being connected to the coil 132 shown in FIG.15. The control unit 140 of the reading device is programmed to applythe interrogating field in pulses with listening intervals between thepulses and the coil 270, 272 acts both to generate the interrogatingfield and to pick up the regenerated fields from the magnetostrictiveelements during the listening intervals.

Further, the embodiment of FIGS. 23 to 27 includes an identity tag 280(FIG. 23) mounted on the structure 214 enabling the identity of themeter to be automatically determined and recorded by the apparatus shownin FIG. 15. The tag 280 comprises a hollow tray 282 made of syntheticplastics material or other magnetically transparent material containinga magnetostrictive element 12a similar to the element 12 of FIG. 1 and ahard magnetic element 14a, similar to the element 14 of FIG. 1, issecured to and forms a cover for the tray 282. The element 12a is freeto vibrate in the tray 282 and the element 14a is encoded in the mannerdescribed with with reference to FIGS. 1 to 9 with a magnetic patternwhich biases the element 12a to resonate at a combination of itsharmonic frequencies or its fundamental and harmonic frequencies chosento indicate the identity of the particular meter.

Various coding arrangements for representing the identity of the metersby means of tags 280 are possible. For example, the different digits ofa binary number might be represented by resonance at differentharmonics. Thus, for example, the digits of a four bit binary numbermight be represented respectively by resonances at twice, three times,four times and eight times the natural frequency of the magnetostrictiveelement. The presence of a resonance might indicate binary 1 and absencebinary 0. However, whilst this arrangement will be satisfactory forbinary numbers having relatively few digits, signal to noise ratio willtend to decrease as the number of digits is increased if this codingarrangement is used. An alternaitve coding arrangement which would givenumbers from 0 to 219 would be to record only three harmonics (with thefundamental frequency excluded as such frequency carries the highestrisk of being excited accidentally). Thus, there are 220 possiblecombinations of 3 out of 12 harmonics. This coding system would almostequate to an eight bit binary number (which can give 256 combinations)but with a much higher signal to noise ratio and therefore much higherreliability than would be obtained if up to eight harmonics were to berecorded simultaneously on each tag. An advantage of this system is thatif more or less than three resonances are detected at any given timethis suggests that there is a malfunction. Thus, the decoding apparatusused with this system of representing data may be programmed to generatean alarm in response to the detection of an incorrect number ofresonances.

In practice, as there may well be very large numbers of meters to beread, each identity tag would comprise a number of magnetostrictiveelements thus to provide a sufficiently large number of bits in the codeto enable each meter to be uniquely identifiable.

During interrogation, each frequency of the interrogating field may betransmitted in turn with a listening interval between. Alternatively,the frequency of the interrogating field could be swept through allrelevant frequencies and there would thereafter be a listening intervalfor listening for all frequencies. This will normally only be practicalwhen relatively few frequencies are used as the sweeping must, on theone hand, be slow enough to ensure that there is sufficient time for aresponse to be generated at each frequency and, on the other hand, thesweep must be completed in a time sufficient that all elements willstill be resonating after the sweep has been completed. As a furtheralternative, an interrogating field having components at all therequired frequencies could be transmitted simultaneously followed by alistening interval. If desired, reading may be obtained by repeating theelectromagnetic reading operation a number of times for noise reductionor error detection.

By way of example, the magnetostrictive elements in the array 224 mayvary in the length from between 5 mm and 10 mm and may have resonancesin the range 225 kHz to 450 kHz.

The coding system shown in the above Table 2 ensures that at least oneresonance arises for all dial positions. Thus, absence of response tothe interrogating field represents a fault.

The embodiment shown in FIGS. 28 and 29 is identical to that of FIGS. 24to 27 except for the interrogating coil arrangement. In this embodiment,the coil 130 is divided into two parts 180 and 182, one at each end ofthe set of dials 202 to 212, the coil portions 180 and 182 beinggenerally coaxial with the dials. Any of the reading sequences describedabove may be employed.

Linear Encoders

FIG. 30 diagrammatically illustrates a linear encoder embodying theinvention. This comprises a linearly movable elongate strip 300 whoseposition is to be monitored or detected, a stationary transducer unit302 positioned adjacent the strip 300 for monitoring the positionthereof, and a control and display unit 304 connected to the transducerunit 302 for processing signals therefrom and producing a display ofposition of the strip 300.

The transducer unit 302 comprises a housing 306 containing twomagnetostrictive transducers 308 and 310. The transducer 308 ispositioned above a coarse indicator track 312 provided on strip 300 andextends transversely of the strip and transducer 310 is positioned abovea fine indicator track 314 provided on the strip 300 and extendslongitudinally of the strip 300. The transducer 308 comprises amagnetostrictive element 316 within a magnetically transparent casing318 and free to vibrate therein, and a coil 320 which is connected bywires 322 to the unit 304. Similarly, the transducer 310 comprises amagnetostrictive element 324 within a magnetically transparent casing326 and free to vibrate therein, and a coil 328 connected by wires 330to the unit 304.

The track 312 comprises a series of hard magnetic segments or elements332 having different magnetic patters recorded thereon in a mannersimilar to FIG. 12. Thus, the segments 332 may be separate elementsattached to the belt 300 or may be constituted by successive portions ofa single continuous strip of magnetic material on the belt 300. Themagnetic patterns on the segments 332 co-operate with themagentostrictive element 316 such that the element 316 will, in responseto interrogating fields of appropriate frequencies created by the coil320 under control of the unit 304, resonate at different combinations ofits fundamental and/or harmonic frequencies according to a codeindicative of the position of the element 300. In this way, a coarse butabsolute indication of position of the strip 300 is given.

The track 314 is also a hard magentic material and has recorded thereona magnetic pattern 339 produced by applying to a recording head (such asthat shown in FIG. 3) a high frequency sine wave signal 340 (FIG. 31).The pattern 339 extends along the track continuously from one end to theother and consists of successive magnetised zones producing bias fieldsin opposite directions parallel to the length of the strip 300 as shownin FIG. 30. The frequency of the signal 340 is, relative to the lengthof the magnetostrictive element 324 (in a direction parallel to thestrip 300) such as to bias the element 324 to resonate at a high orderharmonic in response to an interrogating field of appropriate frequencyapplied by the coil 328. For example, the element 324 might have alength of 20 mm and the signal 340 might have a wavelength of 2 mm so asto bias the element 324 to resonate at its 20th harmonic.

FIG. 31 illustrates diagrammatically the signal 340 extending along thestrip 300 and indicates by reference numbers 324a to 324i nine examplesof the possible positions of the end of the element 324 relative to thesignal 340. At position 324a, the end of the element 324 is coincidentwith a zero crossing point of the signal 340 and, as indicated at 350,the element 324 will resonate at a single harmonic (the 20th in theexample given above) in response to the interrogating field of thisfrequency. This is true of each zero crossing point and thus is alsotrue for the position 324i shown in FIG. 31. When the end of the element324 is coincident with a maximum or minimum of the signal 340, forexample as shown an 324e, the magnetostrictive element 324 does notresonate at the harmonic 350, but instead resonates at harmonics 352 and354 below and above the harmonic 350 respectively. As can be seen fromFIG. 31, as the relative position between the element 324 and the signal340 moves from the position 324a to 324e, the resonance indicated at 350gradually decreases and the resonances indicated at 352 and 354gradually increase. There is a position at 324c where all threeresonances are of equal amplitude which corresponds to about 60 degreesof the sine wave. Between 60 and 90 degrees, the resonances 352 and 354become larger than that at 350. Upon movement from the 90 degree(maximum) position of the wave 340 to the next zero crossing point, theresonances at 352 and 354 decrease in amplitude and the resonance at 350again increases in amplitude until at the next zero crossing point 324i,the resonance at 350 only remains. This is repeated as each half cycleof the recorded signal 340 moves past the magnetostrictive element 324.Thus, by applying appropriate interrogating frequencies to the coil 328,the resonances 350, 352 and 354 can be excited and detected so as todetermine the position of the member 300 with high resolution but withno absolute indication of position. As will be clear from FIG. 31, suchresolution can easily be obtained to within a quarter of the wavelengthor better. Resolution to less than 1 mm or down to perhaps 10 micronswould be achievable.

As shown in FIG. 32, the unit 304 comprises power supplies 360 and 362for energising the coils 320 and 328 respectively, a control unit 364for causing the power supplies to be energised in pulses and at afrequency swept through the range of frequencies selected for theparticular embodiment (or alternatively such frequencies may betransmitted in a burst), decoders 366 and 368 for decoding the signalsgenerated by resonance of the elements 316 and 324 respectively, datastores 370 and 372 for storing data from the decoders and a display 374energised by a display encoder 376 to display the position of theelement 300 under control of the control unit 364. The control unit 364may comprise a microprocessor appropriately programmed.

The decoder 368 for decoding the signal received from themagnetostrictive element 324 may, where the interrogating signal isswept in frequency, operate as follows. The received signal will be(typically) a time-dependent voltage, containing up to three peaks ofvarying amplitude. This voltage signal can be digitised with ananalogue-to-digital converter (ADC), and the resultant numbers stored ina convenient electronic memory (eg a FIFO--first-in-first-out-buffer).The processor unit can then search the memory to identify the peakvalues. This can be done by searching for the three highest values inthe vicinity of the (known) peak positions, or by using numericaltechniques to fit a three-peak curve to the data.

Other approaches can be used if higher speeds are desired. For example,the input voltage can be electronically differentiated and thezero-crossing (corresponding to the peaks of the original signal)identified. This technique is potentially very fast but is susceptibleto noise.

Alternatively, if the magnetostrictive elements are interrogated bypulse-echo means, then the return signal can be passed throughnarrow-band filters to provide three channels. The value of the voltagein each channel then corresponds to the value of the peak signal.

In all cases the value of the position can be determined by a variety ofmeans once the values of the peaks are known. For example, the ratio ofthe centre peak height to the heights of the "wing" peaks can be taken(once any background has been removed and provided care it taken toavoid "divide by zero"), and a look-up table used to convert this ratioto a measure of position.

FIG. 33 is a diagram similar to FIG. 31 but shows a modification to theembodiment of FIG. 30 in which, in addition to the high frequency signal340, recorded on the hard magnetic track 314, a lower frequency signal380 is also recorded thereon, the composite of these two signals beingindicated at 382 in FIG. 33. As shown at 384 in FIG. 33, the highfrequency signal 340 produces resonances 350, 352 and 354 dependent uponthe position of the element 324 with respect to the high frequencysignal 340. As indicated at 386, the low frequency signal 380 producesresonances 350a, 352a and 354a, the resonance 350a appearingsubstantially alone when the end of the element 324 is coincident with azero crossing point in the wave 380 and the resonances 352a and 354aappearing subtantially alone when the end of the element 324 iscoincident with a maximum or minimum of the wave 380. By providing thelow frequency signal 380 in addition to the high frequency 340, thedistance through which the member 300 has to move before the resonances384 and 386 repeat is increased compared to the arrangement shown withreference to FIG. 31.

FIG. 34 diagrammatically illustrates an embodiment in which themagnetostrictive element 400 is positioned between concentric coils 402and 404, and a strut 406, whose linear position is to be detected, ismovable axially through the coil 404 as indicated by arrow 408. Amagnetic pattern 410 is recorded on the strut 406 for biasing theelement 400 to resonate at different combinations of its fundamentaland/or harmonic frequencies in the manner previously described. Thecoils 402 and 404 are for applying an interrogating field to the element400 and detecting the regenerated field which arises upon resonance.

The winding of the coils 402 and 404 is such that, as shown by thearrows in FIG. 35, the fields they produce add in the annular space 412between the coils and between which the element 400 is located butcancel in the region inside the coil 404 and the region outside the coil402. In this way, unwanted stray magnetic fields produced uponenergisation of the coils can be reduced or eliminated and sensitivityof the coils to unwanted stray electromagnetic fields may besubstantially reduced or eliminated, such sensitivity otherwise arisingsince the strut 406, being of or comprising magnetic material, wouldtend to cause a single coil surrounding it to act as an antenna in whichunwanted oscillations would be generated by unwanted strayelectromagnetic fields.

One application for the device shown in FIGS. 34 and 35 is for detectingmovement of a suspension strut in a motor car as is required, forexample, in active suspension systems where suspension elements aredriven in a feed back system.

In the illustrated embodiments, known magnetostrictive materials may beused for the magnetostrictive elements. Examples are amorphous,spin-melt ribbon such as sold under the trade mark "METGLAS 2605" orgrain-oriented silicon transformer steel. The material chosen preferablyhas a high magnetic permeability with a high magnetostrictive coupling.The hard magnetic elements may be made of any of a variety of hardmagnetic materials. Examples include magnetic stainless steel, nickel,ferrite or mild steel. Alternatively, the hard magentic material maycomprise a non-magnetic substrate having a magnetic coating thereon,such as slurry-formed ferrite as used in magnetic tapes and magneticdiscs. The properties required for the biasing element are that itshould be capable of being magnetised substantially permanently.

Modifications

A number of modifications which may be made in the above embodimentswill now be described with reference to FIGS. 36 to 52.

As has been described above, the natural frequency of the rectangularmagnetostrictive elements illustrated is dependent upon the lengththereof and, where magnetostrictive elements having different naturalfrequencies have been described, they have been shown as havingdifferent lengths, for example, as in FIG. 14. Alternative ways ofachieving different natural frequencies are illustrated in FIGS. 36 to39.

FIG. 36 illustrates a magnetostrictive element 512 which may be utilizedin a variety of embodiments of the invention. The element 512, which maybe stamped or etched from a sheet of magnetostrictive material,comprises a strip 514 of length l with four side projections 516adjacent its ends. The effect of the projections 516 is to reduce thenatural frequencey that the strip 514 of length l would otherwise have.This reduction in natural frequency arises from the addition to thestrip 514 of the mass associated with the extensions 516. Thus, for agiven required natural frequency, the dimension l may be reduced. As inthe previously described embodiments, a hard magnetic biasing memberhaving an appropriate magnetic pattern recorded thereon will beassociated with the element 512. The size and shape of the hard magneticbiasing member may, for example, be the same as the strip 514 asindicated by dotted lines 520 in FIG. 36 or, as an alternative, might bea rectangle whose size is equal to the outline shape of the element 512as indicated by dotted lines 522 in FIG. 36. As a further alternative,the size of the hard magnetic biasing member might be intermediate thesizes indicated by lines 520 and 522.

FIG. 37 shows a magnetostrictive element 512 similar to that shown inFIG. 36 except that, instead of rectangular projections 516, projections518 of trapezoidal shape are shown. Thus, if the area of the projections518 is less than that of the projections 516 but the elements shown inFIGS. 36 and 37 are otherwise the same, the natural or fundamentalfrequency of strip 514 of FIG. 37 will be somewhat higher than that ofstrip 514 of FIG. 36 due to the lower mass of projections 518 comparedto projections 516. A hard magnetic biasing member as described withreference to FIG. 36 may be used with the element 512 shown in FIG. 37and thus the same reference numbers in FIG. 37 designate items whichcorrespond to those described with reference FIG. 36.

FIG. 38 shows a further modified magnetostrictive element 512 similar tothat described with reference to FIGS. 36 and 37 and accordinglycorresponding reference numbers are used to indicate correspondingitems. In FIG. 38 lateral projections 524 are provided on the strip 514instead of the projections 516 and 518 of FIGS. 36 and 37. Eachprojection 524 is of generally L-shape and the projections thus formE-shapes with the strip 514. Apart from the shape of the projections524, the description given with reference to FIGS. 36 and 37 alsoapplies to FIG. 38. As a specific example of frequencies obtainable witha device as shown in FIG. 38, i might have a value of 5 mm which, in theabsence of the projections 524, would mean that the strip 514 would havea fundamental frequency of 440 KHz. The mass of the elements 524 mightbe such as to reduce this frequency to a lower value of say 113 KHz and,in an experimental set up, it has been found that a hard magneticelement having a width slightly greater than that indicated by dottedlines 520 and producing a biasing field to induce resonance of the strip514 at its fundamental frequency will produce an additional unwantedresonance at a higher frequency (in the experiment this was found to be223 KHz), in response, of course, to an interrogating field of therelevant frequencies. This unwanted frequency will be taken into accountin designing any practical system.

Elements with side projections such as those shown in FIGS. 36 to 38 toprovide different natural frequencies whilst maintaining the length lconstant may accordingly be used instead of the elements of differentlength shown in FIG. 14.

Similarly elements of the shapes illustrated in FIGS. 36 to 38 may beemployed for providing elements of different natural frequency in anarray similar to that shown in FIG. 26 instead of providing elements ofdiffering lengths. FIG. 39 illustrates part of such an array in which,as shown, five elements 512a to 512e are formed from a common piece ofmaterial and remain connected by a support strip 513. All elements havethe same length l but, with their various projections, are otherwise ofdifferent shapes and sizes to provide different natural frequencies.Thus, an array comprising elements similar to those shown in FIG. 39 maybe used in the embodiment of FIG. 23 in place of the array shown in FIG.26.

FIG. 40 illustrates a modification to the linear encoder shown in FIG.30. As shown in FIG. 40, in addition to the resonator 310 whichcooperates with the track 314, a further similar resonator 310' isprovided also cooperating with the track 314. If the resonators 310 and310' are to be excited simultaneously they should have different naturalfrequencies so that their resonances are distinguishable. This can beachieved by making them of slightly different lengths but preferablythey have the same length as each other in which case the difference innatural frequencies can be achieved in the manner described withreference to FIGS. 36 to 38 or otherwise adding mass to one or both ofthe resonators. The resonator 310' is longitudinally offset relative tothe resonator 310 so that when the end of the resonator 310 iscoincident with an zero crossing point of the signal 340, the end ofresonator 310' is part-way between a minimum of the curve 310 and thenext zero crossing point. Thus, FIG. 40 illustrates the resonators 310and 310' longitudinally aligned with each other and spaced from eachother although of course other positions are possible provided theappropriate "phase" difference with respect to the recorded signal 340is maintained.

The letters A, B, C, D, E and F marked on the horizontal axis upon whichcurve 340 is drawn in FIG. 40 indicate examples of six positions of theend of resonator 310 relative to the recorded signal 340 as the track314 is moved therepast, positions A and E being zero crossing points andposition C being a maximum point. The waveforms in the lower part ofFIG. 40 show the resonances which will be produced in the resonators 310and 310' (in response to interrogating fields of appropriate frequency)when the track is in the positions A to F respectively. Since the mannerin which the resonance changes as the track 314 moves past the resonator310 has already been described with reference to FIGS. 31 and 33, it isbelieved that FIG. 40 will be readily understood without furtherdescription.

The purpose of the additional resonator 310' illustrated in FIG. 40 isto extend the distance through which the track 314 must move before theresonances begin to repeat, the position of the resonantors 310, 310'relative to the track 314 thus being unambiguous through points A to E.As will be appreciated, instead of the simple sinewave signal 340 shownin FIG. 40, composite signals may also be used on the track 314.

FIG. 41 illustrates a modification to the embodiments of FIGS. 23 to 29in which five magnetostrictive resonators are associated with each dialinstead of four and each dial carries a hard magnetic biasing elementextending over five contiguous digit positions of the dial i.e. half-wayround the circumferance. Thus, FIG. 41 diagrammatically illustrates oneof the dials indicated by reference number 600, five magnetostrictiveresonators 602, 604, 606, 608 and 610 each having a different naturalfrequency from the others and spaced from each other so as to beopposite respective different digit positions. Hard magnetic biasingelement 612 extending half-way round the dial 600 is alsodiagrammatically illustrated. The dial 600 may be similar to the dialsillustrated in FIGS. 23 to 29 and the magnetostrictive elements 602 to610 and the hard magnetic element 612 may be mounted in a manner similarto the corresponding elements in those figures. Similarly, the dial 600may be positioned in a casing 614 having a viewing window 616 throughwhich the numbers of the periphery of the dial are visible.

If the hard magnetic material 612 extends over digit position zero tofour inclusive, the sequence of codes for the successive dial positionsmay be as set out below in Table 3.

                  TABLE 3                                                         ______________________________________                                        Dial number                                                                              a         b     c       d   e                                      ______________________________________                                        0          0         0     0       1   1                                      1          0         0     0       0   1                                      2          0         0     0       0   0                                      3          1         0     0       0   0                                      4          1         1     0       0   0                                      5          1         1     1       0   0                                      6          1         1     1       1   0                                      7          1         1     1       1   1                                      8          0         1     1       1   1                                      9          0         0     1       1   1                                      ______________________________________                                    

In the above Table, the columns headed a to e represent respectively theresponse of the magnetostrictive elements 602 to 610, with a "0"indicating that the element is not biased to resonate and a "1"indicating that it is biased to resonate in response to an interrogatingfield of appropriate frequency.

FIG. 42 shows a further possibility for the mounting of theinterrogating coil for interrogating the positions of the dials in, forexample, the embodiment of FIG. 23. Thus, in FIG. 42, reference 700indicates three of the dials included in the meter and reference 702indicates an interrogating coil (without showing any mounting meanstherefor) wound coaxial with the dials 700. As can be seen, the lowerportions of the coil 704 extend round the dials 700 whereas the upperportions 706 are offset this being to avoid interfering with thevisibilty of the numbers on the perifery of the dials. Although coil 702is shown as a single strand in FIG. 42 in practice it may comprise anumber of windings. Energization of the interrogating coil 702 may beeffected in any of the various way described herein in relation to otherinterrogating coils. As will be appreciated, although the resonators arenot shown in FIG. 42, these will be positioned between the portions 704of coil 702 and the dials 700.

FIGS. 43 to 46 illustrate diagrammatically means by which the responseof the magnetostrictive element to the interrogating field may beenhanced, such means operating by causing the flux in the interrogatingfield to be concentrated in the region containing the magnetostrictiveelement. Such means may be applied, with appropriate structure, to anyof the magnetostrictive elements referred to above where, in a givenapplication, there is sufficient space for doing so. For illustrativepurposes, however, the response enhancing means will be described withreference to FIGS. 43 and 44 as applied to an indentification elementsuch as element 280 illustrated in FIG. 23 but in which theidentification element includes a number of magnetostrictive resonatorshaving different fundamental frequencies and biased to resonate atvarious harmonics whereby a multi-digit number indicative of theidentity of the meter may be encoded.

Thus, FIGS. 43 and 44 show a set of five magnetostrictive resonatorelements 812 each having an associated hard magnetic biasing element 814positioned between planar soft magnetic sheets of high permiability 830and 832 of trapezoidal shape. As can be seen in FIG. 44, the sheets 830and 832 are in the same plane as the elements 812. The narrow ends 834and 836 of the sheets 830 and 832 respectively are positioned adjacentthe ends of the elements 812 and the wide ends 838 and 840 of the sheets830 and 832 respectively are accordingly spaced from the elements 812.As seen in FIG. 43, the sheets 830 and 832 are of different shape fromeach other so that the edges which are adjacent the ends of the elements812 are located as closely as possible thereto. The effect of thisarrangement is to concentrate the magnetic flux of the interrogatingfield in the region between the narrow ends 834 and 836 of the sheets830 and 832, that is to say in the region containing themagnetostrictive elements 812. This effect is shown by dotted lines 842in FIG. 43, representing the flux lines of the interrogating field. As aresult of this, the sensitivity of the magnetostrictive elements to theapplied field is increased. Thus, the range of operation is increasedfor a given power level. Alternatively, this arrangement will allowlower power levels to be used for the interrogating field if the rangeis maintained or, as a further alternative, if both range and powerlevels are maintained, enhanced signal to noise ratio will be achieved.

A suitable magnetic material for the sheets 830 and 832 isVacuumschmelze 6025.

FIGS. 45 and 46 show an alternative arrangement for concentrating theflux of the interrogating field in the region containing themagnetostrictive element 812. In this case, only a singlemagnetostrictive element 812 is shown in the drawings, although ofcourse a set of such element may be provided if desired. In FIGS. 45 and46, a rectangular sheet 844 of non-magnetic material, such as aluminium,has an aperture 846 in which the element or elements 812 and 814 arelocated, with the sheet 844 extending generally perpendicularly to theelements 812 and 814 and positioned at about the centre thereof. Anarrow gap 843 extending from the aperture 846 to the edge of the member844 ensures that there is no short circuit path extending all the wayaround the gap 846. As represented by broken lines 848 in FIG. 46, thelines of flux of the interrogating magnetic field pass around the sheet844 and through the aperture 846 therein, those passing through theaperture 846 thus being concentrated in the manner shown and thusenhancing the strength of the interrogating field in the region of theopening 846 where the magnetostrictive element 812 is located. To beeffective, the sheet 844 should have a thickness at least as great asthe electromagnetic skin depth. Optimally, therefore, the thickness isslightly greater than this depth so as to achieve the required resultwithout wasting material.

In the embodiments so far described, the magnetic field patternsprovided in the hard magnetic element have been such as to bias themagnetostrictive elements to resonate longitudinally. That is to say therectangular magnetostrictive strips stretch and contract in thelongitudinal direction in response to the applied interrogatingalternating field of appropriate frequency. It is possible, however,within the scope of the invention to provide biasing fields which causenon-longitudinal modes of vibration. Two examples will be described withreference to FIGS. 47 to 52.

With reference to FIG. 47, a magnetostrictive element 912 is biased by ahard magnetic biasing element (not shown) in such a manner that thefields in the upper and lower parts of the strip element 912 aredirected in opposite longitudinal directions as shown by arrows 914 and916. FIG. 48 shows at Aa and Ab respectively the signals recorded on thebiasing element along lines a--a and b--b indicated in FIG. 47. Theeffect of this magnetic pattern is that, in response to an interrogatingfield of appropriate frequency, the upper and lower portions of thestrip 912 as shown in FIG. 47 will extend and contract in anti-phase toeach other producing flexural vibrations of the strip in its own planeas shown in chain dotted lines in FIG. 49. The frequency at which theseoscillations occur will differ from the fundamental frequency of theelement 912. In order to produce other vibrational modes at otherfrequencies, other signal patterns may in practice be superimposed onthose shown on FIG. 48, for example for causing the element 912 tovibrate longitudinally at harmonics of its fundamental frequency in themanner described with reference to FIGS. 1 to 9.

In the embodiment of FIG. 50, a magnetostrictive strip element 918 isbiased by a hard magnetic element (not shown) producing a fieldtransverse to the length of the strip as shown by arrows 922. Thestrength of the field represented by arrows 922 is greatest at the endsof the strip and decreases towards the centre of the strip at whichregion the field is substantially zero. Thus, FIG. 51 illustrates at Ba,Bb and Bc the signals recorded on the magnetic biasing element along thelines aa, bb and cc respectively as marked on FIG. 50. It should beunderstood that the horizontal axis in the graphs of FIG. 51 representsthe signal recorded in a direction transverse to rather thanlongitudinally of the strip 922. The effect of this magnetic fieldpattern is to produce transverse vibrational distortion of the strip ina manner somewhat as shown in FIG. 52 where, essentially, the ends ofthe strip are splayed transversely in response to an interrogatingalternating field of appropriate frequency. As will be seen in FIG. 52,there is some apparent contraction in the width of the strip in theintermediate zones bb and cc. This has been predicted by a computersimulation of the vibration that would be produced by the patterns shownin FIGS. 50 and 51. The frequency of the signal necessary to produce theoscillations shown in FIG. 52 will be different from the fundamentalfrequency of the strip. As with the other embodiments, other fieldpatterns may be superimposed on those shown in FIGS. 50 and 51 so as toarm the strip for resonants at other frequencies, such as itsfundamental frequency and/or harmonics thereof.

Thus, the embodiments of FIGS. 47 to 52 are illustrative of theprinciple that, in accordance with the invention, vibration in differentmodes may be induced in the strip to provide additional frequencies towhich the strip will respond.

Many further modifications are possible within the scope of theinvention. For example, instead of the arrangement shown in FIG. 15,meter reading could be achieved by placing the reading device close tothe meter. However, the provision of the coils 130 and 132 as shown inFIG. 15 makes it possible, for example, for the meter to be locatedinside a house or underground and reading to take place from a fewmeters away.

In another modification, a single magnetic biasing member could bepositioned next to the or each dial and magnetostrictive elements couldbe mounted on the dial at positions corresponding to the numbers thereofso as to be biased to resonate at their natural frequencies whenpositioned next to the biasing element. The magnetostrictive elementswould be mounted in cavities slightly larger than the elements so thatthe elements are free to vibrate. Magnetostrictive strips havingdifferent natural frequencies would be associated with the differentnumbers so that the numbers can be distinguished from each other. Wherethe relative position of two linearly moveable members is to bedetected, a row of magnetostrictive elements could be provided along oneof the members, each element being adapted to resonate at a differentfundamental frequency, and a hard magnetic biasing element would beprovided on the other element so as to move along the row ofmagnetostrictive elements and bias different ones thereof as the twomembers linearly move relative to each other.

It would also be possible to construct a device in accordance with theinvention for indicating the value of a variable without any movingparts. Electrical means could be provided for producing differentmagnetic patterns as by energising a set of electromagnetic coils as afunction of the value of the variable and a magnetostrictive element orelements biased by said magnetic pattern could be arranged to resonateat different frequencies according to the magnetic biasing patternproduced. Thus, the different frequencies or combinations of frequencieswould be indicative of the value of the variable.

Although the members 124 in the embodiment of FIG. 10 have beendescribed as separate hard magnetic strips, it would alternatively bepossible to provide a single hard magnetic coating on the surface ofeach drum so that the "strips" would not be physically separate butwould merely be zones with differing magnetic patterns recorded in them.This also applies to the elements 124' and 124" of the otherembodiments.

Although in the embodiments of FIGS. 23 to 29, the array 24 ofmagnetostrictive elements has been described as being formed by etchinga single sheet of magnetostrictive material, such an array could be madein other ways. For example, the magnetostrictive elements could beindividually made and attached to a magnetically transparent supportingsheet such as a sheet of plastics material (for example Mylar) whichwould be mounted adjacent the dials in a manner similar to the array224. Further, although only two hard magnetic elements 226a and 226bhave been shown in the embodiments of FIGS. 24 to 29, each extendingover a number of digit positions, it would be possible instead toutilize a separate hard magnetic element for each of the digit positionswhich is to have a hard magnetic element associated therewith.

Although in the description with reference to the drawings, themagnetostrictive strips have been biased in only one direction, i.e.along their length, it would also be possible to bias them transverselyby appropriate field patterns so that they will be caused to resonate inthe transverse direction instead of or in addition to resonance in thelongitudinal direction.

Although, in the above description, there have been various referencesto causing the interrogating field to be swept or stepped through therequired range of frequencies, other alternatives are possible. Forexample, the interrogating field could comprise a burst of the requiredinterrogating frequencies generated simultaneously or, in somesituations, could be in the form of a burst of noise, such as whitenoise, containing a large number of frequencies in addition to thoserequired for causing resonance.

In the embodiment of FIG. 30, it would be possible to omit the track 314and transducer 310 where only a coarse indication of absolute positionis needed. Alternatively, the track 312 and transducer 308 could beomitted and reliance place upon the high resolution track 314. In thatcase, where an absolute indication of position of the member 300 isneeded, rather than merely a measure of incremental movements thereof,an appropriate alternative means for giving this information could beprovided, for example as by moving the member 300 to one end positionand then keeping a count of incremental movements therefrom.

It has been described with reference to FIGS. 36 to 39 that theprovision of projections at the ends of the strip reduces thefundamental frequency of the strip. Alternative ways of reducingfundamental frequency are possible, in particular by adding mass in someother way such as by depositing massy material at appropriate positions.

Although with reference to FIGS. 37 to 52, it has been indicated thatthe magnetostrictive element may be biased to resonate in differentmodes or directions, in many applications it will be desired to ensurethat resonance in only the longitudinal mode is achieved. In suchapplications, a simple rectangular strip of magnetostrictive materialmay be used with a high aspect ratio i.e. the strip is long and thin sothat longituninal vibration is maximized and transverse vibrationminimized. For a constant length of strip, reducing the width reducesthe total signal because the amount of material is reduced but improvesthe signal to noise ratio because the resonance produced has asubstantially narrower frequency band. If the length of the resonator isan integer multiple of its width then difficulties may arise in usingcertain harmonics. For example, in experimentation with devices having a3:1 aspect ratio, the third harmonic (i.e. a frequency three times thefundamental) was not usable because it was split into a doublet. Asimilar effect occurred at the ninth harmonic with this arrangement.Further, the resonant frequencies were not exact multiples of thefundamental where low aspect ratio is used and this is believed to bedue to the excitation of resonances transverse to the strip. Theseproblems may be avoided by using high aspect ratio strips, for examplean aspect ratio of 15:1 or more, since the problems under discussionwill then only arise at very high harmonics (frequencies of 15 times thefundamental or higher where the aspect ratio is 15:1), which would beoutside the range of frequencies used.

It has been described above with reference to FIGS. 3 to 9 that thesignal source 20 is preferably turned on before the element 14 reachesthe transducer 18 and is turned off after the element 14 has passed forthe purpose of avoiding transients. In many situations, however, thiswill not be necessary since careful design of the system can avoid theoccurrence of unwanted transients.

The invention has a number of advantages. It permits the remote readingof variable data such as that provided by the dials of water and gasmeters by means which are compatible with the existing configurations ofsuch devices and which permit the data to be read in the ordinary waywithout interference with visibility. The invention has the furtheradvantages of low cost, intrinsic safety and ruggedness and it imposesno mechanical drag of moving parts. Being passive, it has a long workinglife without maintenance or replacement of parts.

We claim:
 1. Measuring apparatus for generating a multidigitrepresentation of a value of a variable, said apparatus comprising:aplurality of movable members, each corresponding to a respective one ofsaid digits and movable to different positions in response to variationin the value of said variable so that the positions of said members areindicative of the values of said digits; a plurality of magnetic fieldgenerators associated respectively with said respective movable members,each said generator generating an alternating magnetic field having aparameter which varies as a function of the position of the respectivemovable member, said alternating fields being at different frequenciesfrom each other such as to be distinguishable from each other; anexciting signal generator coupled in common to said magnetic fieldgenerators for applying thereto exciting signals at said differentrespective frequencies; and an output signal generator coupled in commonto said magnetic field generators for producing a plurality of outputsignals, each corresponding to a respective one of said generatedalternating magnetic fields and each having a value dependent upon thevalue of said varying parameter of the corresponding alternatingmagnetic field.
 2. Measuring apparatus according to claim 1, whereinsaid movable members rotate to different angular positions in responseto variation in the value of said variable.
 3. Measuring apparatusaccording to claim 1, wherein each alternating magnetic field has anamplitude, and wherein said parameter is the amplitude.
 4. Measuringapparatus according to claim 1, wherein each alternating magnetic fieldhas a frequency, and wherein said parameter is the frequency. 5.Measuring apparatus according to claim 1, wherein each magnetic fieldgenerator comprises a magnetostrictive element and a bias element forbiasing said magnetostrictive element to resonate when subjected to saidexciting signals.
 6. Measuring apparatus according to claim 1, whereinsaid common exciting signal generator and said common output signalgenerator are formed as a single coil.
 7. Measuring apparatus accordingto claim 1, wherein said plurality of movable members and said pluralityof magnetic field generators are provided in a first unit and saidcommon exciting signal generator and said common output signal generatorare provided in a second unit, said first and second units beingseparate.
 8. Measuring apparatus according to claim 3, wherein saidamplitude varies in an analog manner as a function of the position ofthe respective movable member.
 9. Measuring apparatus according to claim3, wherein said amplitude varies in a digital manner as a function ofthe position of the respective movable member.
 10. Measuring apparatuscomprising:a plurality of measuring members, each being movable inresponse to variation in a variable to be measured to positionsindicative of the variable; a plurality of magnetic field generatorsassociated respectively with said respective movable members, each saidgenerator generating an alternating magnetic field, said alternatingfields being at different frequencies from each other such as to bedistinguishable from each other; an exciting signal generator coupled incommon to said magnetic field generators and for applying theretoexciting signals at said different respective frequencies; and an outputsignal generator coupled in common to said magnetic field generators forproducing a plurality of output signals, each corresponding to arespective one of said generated alternating magnetic fields and eachhaving a value dependent upon the position of the correspondingmeasuring member.
 11. Measuring apparatus according to claim 10, whereinsaid measuring members rotate to different angular positions in responseto variation in the value of said variable.
 12. Measuring apparatusaccording to claim 10, wherein each magnetic field generator comprises amagnetostrictive element and a bias element for biasing saidmagnetostrictive element to resonate when subjected to said excitingsignals.
 13. Measuring apparatus according to claim 10, wherein saidcommon exciting signal generator and said common output signal generatorare formed as a single coil.
 14. Measuring apparatus according to claim10, wherein said plurality of measuring members and said plurality ofmagnetic field generators are provided in a first unit and said commonexciting signal generator and said common output signal generator areprovided in a second unit, said first and second units being separate.